T-cell antigen recognition requires antigen presenting cells (APCs) to present antigen fragments (peptides) on their cell surface in association with molecules of the major histocompatibility complex (MHC). T cells use their antigen specific T-cell receptors (TCRs) to recognise with high specificity the antigen fragments presented by the APC. Such recognition acts as a trigger to the immune system to generate a range of responses to eradicate the antigen which has been recognized.
Most of the specificity of T cell recognition of the antigen fragments is provided by a smaller subsequence of amino acids within the fragments. This subsequence is known as the T cell epitope. In the case of extracellular allergens and auto- or allo-antigens, the peptides are presented on MHC Class II molecules, which are recognized by CD4 T cells. Accordingly, interest in allergic and auto- or allo-immune disorders has focused on MHC Class II-binding T cell epitopes.
Given their role in the immune system, there is considerable interest in such epitopes for use as therapeutic agents to modulate the immune systems of subjects. For example, administration of peptide epitopes to subjects has been demonstrated to result in the induction of tolerance to the antigen from which the epitope derives. Therapeutic agents based on such an effect have great potential in the prevention and treatment of allergy, and auto- or allo-immune diseases where the down-regulation of an immune response is desirable.
Further progress in this area is hindered by a number of problems. Firstly, epitope sequences from allergens and auto- and allo-antigens are often poorly soluble, and are therefore problematic both to manufacture and to administer to subjects. Secondly, the majority of epitopes have typically been poorly defined. Most epitopes known in the art are loosely identified as being a core sequence present somewhere within a longer sequence, typically of approximately twenty amino acids. The core sequence itself is often not identified. In the absence of a clear definition of the core sequence an epitope, it has not been possible to modify known T cell epitopes to improve their solubility, since this risks eliminating the core residues required for T cell recognition.